The invention concerns a method and a system of detection of a run-flat condition of a vehicle tire as well as tires, wheels and safety inserts designed to facilitate that detection.
When a mounted assembly—tire and wheel assembly—contains means of support for the tire tread in case of running flat, which means of support make it possible to avoid a forced stop of the tire in case of serious loss of air pressure in the tire. Those means of support can be a safety insert placed radially outside the rim of the wheel of the mounted assembly or reinforcing elements placed inside the structure of the tire sidewalls and/or beads. Such tires are called “self-supporting” tires.
The bearing of the tire on those means of support is accompanied by a more or less marked degradation of its performance, which may not be perceptible to the driver through the behavior and comfort of the vehicle. Furthermore, the operating lifetime of those means of support is limited. It is therefore useful for the driver to be alerted as soon as a tire bears on its means of support so that he can follow the manufacturer's instructions.
Patent application WO 94/03338 proposes a system of detection of the bearing of a tire on a safety insert. That system comprises one accelerometer per wheel, placed on one of the suspension elements of the wheel and measuring the vertical accelerations linked to a central processing unit. The analysis is based on detection of the appearance, upon bearing, of a resonance mode of the system consisting of the tire bearing on the safety insert, of the unsuspended weights and of the suspension springs. That resonance mode is characteristic of running flat and is situated above 100 hertz.
However, for some safety supports, made, for example of elastomeric material, the sensitivity of the aforesaid analysis may prove insufficient.
In the case of mounted assemblies not containing the aforesaid means of support, on a run-flat condition, the tread bears on the beads and the rim flanges. That can result in a rapid deterioration of the tire and movement of the beads into the rim mounting groove, not to mention degradation in the behavior of the vehicle. As soon as such a support comes into play, it is also very useful to alert the driver.
In what follows, the “run-flat condition” of a tire means running when the air pressure in the tire is no longer sufficient to guarantee that the tire will carry the load of the tire. The tire tread then bears on the support elements. Those support elements can be provided for that purposes (such as safety inserts arranged around the rim or rim flanges).